WASHINGTON: US Secretary of State John Kerry called his Russian counterpart on Wednesday and warned him that Washington will end talks on the Syrian conflict unless Moscow halts the assault on Aleppo.
A State Department spokesman said Kerry told Foreign Minister Sergei Lavrov that plans to set up a joint US-Russia military cell to target jihadist groups in Syria would also be put on hold.
"He informed the foreign minister that the United States is making preparations to suspend US-Russia bilateral engagement on Syria... unless Russia takes immediate steps to end the assault on Aleppo and restore the cessation of hostilities," spokesman John Kirby said.
Kerry and Lavrov have been leading international efforts to bring Syria's five-year-old civil war to an end, and on September 9 agreed to demand a ceasefire.
Moscow was to order its ally Syrian leader Bashar al-Assad to rein in his military and end the bombardment of civilian communities, and Washington was to persuade rebel forces to separate themselves from the jihadist Fateh al-Sham, the former Al-Qaeda affiliate once known as Al-Nusra Front.
But fighting continued and the truce collapsed.
The US-led coalition bombed a Syrian military base, saying later that it had been targeted in error by warplanes hunting the Islamic State group.
Three days later a UN aid convoy was hit in an air strike that Washington and its allies blamed on Russian and regime forces.
Kerry and Lavrov, long-time sparring partners, traded blame for the failure in fierce exchanges at the UN Security Council last week, but each then said that the dialogue could continue.
In recent days, however, Assad's forces have launched a major offensive to seize the rebel-held eastern half of the city of Aleppo, where a quarter-of-a-million civilians are trapped.
"Secretary Kerry expressed grave concern over the deteriorating situation in Syria, particularly for continued Russian and Syrian regime attacks on hospitals, the water supply network, and other civilian infrastructure in Aleppo," Kirby said.
"The secretary made clear the United States and its partners hold Russia responsible for this situation, including the use of incendiary and bunker buster bombs in an urban environment, a drastic escalation that puts civilians at great risk," he said.
"The secretary stressed that the burden remains on Russia to stop this assault and allow humanitarian access to Aleppo and other areas in need."